Xylose isomerase (EC 5.3.1.5) is an enzyme that catalyzes the reversible isomerization of D-xylose into D-xylose. It is also called glucose isomerase because of its ability to isomerase D-glucose into D-fructose, and it is therefore widely used in industry for the production of high-fructose corn syrup (HFCS).
The isomerization of glucose reaches an equilibrium which is shifted towards fructose at high temperatures. Presently, the industrial isomerization process is performed at temperatures of about 58.degree. to 60.degree. C., using moderately thermostable xylose isomerases, to produce 40-42% fructose syrup. An additional step of chromatography allows the production of 55% fructose rich HFCS, which has a higher sweetening power than sucrose.
Performing the isomerization at 90.degree. or 95.degree. C. would make possible the production of syrups containing 55% fructose, without the last chromatography step, but the half-life of the enzymes used today at those higher temperatures does not permit increasing the temperature of the isomerization reaction. Another advantage to performing the isomerization at high temperatures would be that it would decrease the risks of microbial contamination of the reactor.
Previous attempts have been made to obtain more thermostable xylose isomerases, either by site-directed mutagenesis of moderately thermostable xylose isomerases, or by screening highly thermophilic organisms for xylose isomerase activity. However, none of those attempts have resulted in commercially useful hyperthermostable xylose isomerases.
It obviously would be advantageous to have a hyperthermostable xylose isomerase and an efficient method of producing that enzyme in quantity.